1. Field of the Invention
The present invention relates to a high-frequency switch, and more particularly, to a high-frequency switch used as a built-in component in mobile communication equipment adapting to two frequency-band systems.
2. Description of the Related Art
Portable telephones incorporating two transmission/reception circuits and capable of adapting to a dual band are well known. Current portable telephone systems use, for example, the combination of GSM (operating frequency band: 800 MHz to 1 GHz) and DCS1800 (operating frequency band: 1.7 GHz to 1.8 GHz), the combination of the GSM and PCS (operating frequency band: 1.8 GHz to 1.9 GHz). A high-frequency switch incorporating two switches to switch between the transmission circuit and the reception circuit of each of these two transmission/reception circuits has been proposed.
FIG. 16 is an electric circuit diagram showing a conventional high-frequency switch. For example, the high-frequency switch 1 includes a switch 1a for use with GSM, and switch 1b for use with DCS. To the transmission-side terminals Tx1 and Tx2 of the switches 1a and 1b, the cathodes of diodes D1 and D3 are connected, respectively. The cathodes of the diodes D1 and D3 are grounded via transmission lines 2 and 4, respectively. The anodes of the diodes D1 and D3 are connected to antenna-side terminals ANT1 and ANT2, respectively.
To the antenna-side terminals ANT1 and ANT2, reception-side terminals Rx1 and Rx2 are connected via transmission lines 3 and 5, respectively. To the reception-side terminal Rx1 and Rx2, the cathodes of diodes D2 and D4 are connected, respectively. The anodes of the diodes D2 and D4 are grounded via capacitors C1 and C2, respectively. Voltage control terminals Vc1 and Vc2 are connected to the respective intermediate connection points between the anodes of the diodes D2 and D4, and the capacitors C1 and C2, via resistors R1 and R2, respectively.
In this high-frequency switch 1, the antenna-side terminals ANT1 and ANT2 are each electrically connected to an antenna element 8 via a diplexer which performs the function of mixing or distributing signals of two frequency bands (GSM and DCS for example), and which includes LPF/HPF, BEF/BEF, LPF/BEF, or BEF/HPF, and the transmission-side terminals Tx1 and Tx2 are electrically connected to the transmission circuits (not shown) of GSM and DCS1800, respectively, and the reception-side terminals Rx1 and Rx2 are electrically connected to the reception circuit (not shown) of GSM and DCS1800, respectively. By controlling the voltage applied to the voltage control terminals Vc1 and Vc2, the switch 1a is brought into the transmission mode and the switch 1b is brought into the reception mode during transmission from the switch 1a, while, during transmission from the switch 1b, the switch 1b is brought into the transmission mode and the switch 1a is brought into the reception mode.
In the conventional high-frequency switch 1, however, when the switch 1a is brought into the transmission mode and the switch 1b is brought into the reception mode during transmission from the switch 1a, high harmonic signals such as a second harmonic and a third harmonic of the transmission signal of GSM connected to the switch 1a are transmitted from the antenna element 8.
These high harmonic signals occur because when the switch 1b is in the reception mode, the two diodes D3 and D4 are in the off-state. However, since the point X1 shown in FIG. 16 is connected to the ground via not only the transmission lines 4 and 5 and capacitor C2, but also the diodes D3 and D4 in the off-state, the voltage at the point X1 fluctuates. Once the voltage at the point X1 fluctuates, the diodes D3 and D4, which are non-linear elements, vary in capacitance. If a transmission signal flowing through the switch 1a leaks to the switch 1b, high harmonics occur from this transmission signal due to a variation in the capacitance. In particular, the second harmonic of a GSM transmission signal is readily transmitted from the antenna element 8, since the transmission frequency of the second harmonic of the GSM transmission signal overlaps with the DCS1800 transmission frequency.
To overcome the above-described problems, preferred embodiments of the present invention provide a high-frequency switch having a low occurrence of high harmonic signals.
The high-frequency switch in accordance with a preferred embodiment of the present invention includes a first switch for use with a first transmission/reception band, the first switch having at least two switching elements, a second switch for use with a second transmission/reception band using a frequency different from the operating frequency of the first transmission/reception band, the second switch having at least two switching elements, and a mechanism to apply a voltage between the two switching elements of one of the first and second switches when the other of the switches is in a state of the transmission mode. Herein, as the voltage applying mechanism, for example, a resistor electrically connected between the first and second switches may be used.
Preferably, each of the first and second switches includes a first terminal, a second terminal, a third terminal, and a voltage control terminal, a first diode of which the cathode is electrically connected to the first terminal side, and of which the anode is electrically connected to the second terminal side, a first transmission line electrically connected between the second terminal and the third terminal, a second diode of which the cathode is electrically connected to the third terminal side, and of which the anode is electrically connected to the voltage control terminal side, and a second transmission line electrically connected between the first terminal and a ground.
The arrangement is such that each of the first and second switches includes a first terminal, a second terminal, a third terminal, and a voltage control terminal, and a first diode of which the anode is electrically connected to the first terminal side, of which the cathode is electrically connected to the second terminal side, a first transmission line electrically connected between the second terminal and the third terminal, a second diode electrically connected between the third terminal and a ground in the state wherein the anode is electrically connected to the third terminal side, and a second transmission line electrically connected between the first terminal and the voltage control terminal.
With these unique features and arrangements, when one of the first and second switches is in a state of the transmission mode, even though the other switch is in a reception mode, a voltage is applied by the voltage applying mechanism, to a predetermined point between the two switching elements of the other switch, so that the voltage at this point is constant. This prevents the bias voltage of the switching elements of the other switch from fluctuating, which results in greatly reduced occurrences of high harmonic signals.
Furthermore, it is preferable that, in the high-frequency switch in accordance with preferred embodiments of the present invention, on the surface of the laminated body constructed by laminating a plurality of dielectric layers and the first and second transmission lines of each of the first and second switches, the first, second, and third terminals and the voltage control terminals of each of the first and second switches be provided, as well as the first and second diodes as the first and second switches, and resistors as the voltage applying mechanism be mounted.
With these unique features and arrangements, a laminated high-frequency switch having necessary circuits built into a single component is achieved. This eliminates the necessity for matching adjustment components for connecting matching adjustment elements required to assemble components for the two switches.